The present invention relates to a light sensor circuit which is used as a unit pixel of an image sensor and is capable of producing in a photoelectric converting element a photo current (photo current) proportional to a quantity of incident light, converting it into a sensor signal with a logarithmic characteristic and outputting the converted sensor signal.
There has been developed a conventional light sensor circuit having a wide dynamic range, which comprises, as shown in FIG. 1, a photo-diode PD operating as a photoelectric converting element for producing a photo current Ip proportional to the quantity of incident light Ls falling thereon, a transistor Q1 having a logarithmic output characteristic in a weak inverse state for converting the photo current produced in the photodiode into a voltage signal Vpd by using its sub-threshold region characteristic and a charge coupled device (CCD) for outputting a sensor signal by transferring an electric charge in accordance with the voltage signal Vpd (a CCD line sensor having a logarithmically converted characteristic, Japanese Journal of Institute of Image Information and Television Engineers, vol. 49: pp. 169-173 (1995).
FIG. 2 shows an equivalent circuit of the light sensor circuit of FIG. 1.
The light sensor circuit thus constructed requires time to charge and discharge a parasite capacitor Cp of the photodiode and therefore accumulates a remaining charge therein if the quantity of incident light absurdly decreased for instance by changing a state of illumination from bright to dark. An image sensor composed of the above-described light sensor circuits may suffer occurrence of afterglows due to the remaining charges.
In Japanese Laid-Open Publications Nos. 5-219443 (1993) and 7-46489 (1995), there are disclosed image sensors using a number of light sensor circuits each representing a unit pixel, which is capable of producing in a photodiode a photo current proportional to a quantity of incident light falling thereon and converting the photo current into a voltage signal by using a sub-threshold characteristic of a MOS transistor having a logarithmic output characteristic in a weak inverse state and a fixed gate voltage. These image sensors can exhibit a complete logarithmic output characteristic attaining a wide dynamic range. However, these image sensors may suffer the occurrence of an afterglow of a moving bright point on a display screen.
Japanese Laid-Open Patent Publication No. 2000-329616 discloses another embodiment of an conventional MOS transistor type image sensor using a number of light sensor circuits each representing a unit pixel, which circuit comprises, as shown in FIG. 35, a photodiode PD operating as a photoelectric converting element for producing a photo current proportional to a quantity of incident light Ls falling thereon, a transistor Q1 having a logarithmic output characteristic in a weak inverse state for converting the photo current produced in the photodiode into a voltage signal Vpd by using its sub-threshold region characteristic, a transistor Q2 for amplifying the voltage signal Vpd and a transistor Q3 for outputting a sensor signal in accordance with a timing pulse of a readout signal Vs and which circuit is featured by its wide dynamic range obtained by giving the output a logarithmic characteristic, thereby achieving the high sensitivity of detecting a light signal. In addition, the light sensor circuit is provided with a means for changing a drain voltage VD of the transistor Q1 to a value lower than a normal level value for a specified period to remove a charge accumulated in a parasitic capacity C of the photodiode PD to initialize the circuit. The light sensor circuit can thus obtain a voltage signal Vpd corresponding to the quantity of incident light Ls even if the photo current absurdly changed, thereby eliminating the possibility of occurrence of an afterglow of the pixel even at a small quantity of incident light. However, the above-described light sensor circuit requires the provision of a voltage changing-over circuit for changing-over the power source voltage with low impedance step by step in order to change the drain voltage of the transistor to a value lower than the normal when initializing the light sensor circuit. As a result of this, the circuit configuration of the light sensor circuit is complicated.